Autoinjectors and on-body injectors offer several benefits in delivery of therapeutics. One of the benefits can include simplicity of use, as compared with traditional methods of delivery using, for example, conventional syringes.
Many injector systems use coil and other spring structures to provide actuation energy for functions such as needle insertion and fluid delivery. The use of springs can offer benefits of simplicity and low cost, but can have certain limitations.
There is a linear relationship between force and displacement in spring actuators. To provide sufficient energy for drug delivery at the end of plunger stroke, an excessive amount of energy may be input to the system as drug delivery commences.
Further, as higher viscosity drugs are delivered via autoinjectors, the requisite spring forces will likely increase. Springs with higher spring constants transmit more force to the drug product and primary container. Because kinetic energy is proportional to velocity squared, even incremental increases in the spring constant can result in large changes in the net kinetic energy applied to the drug and primary container.
The patient may feel this excessive energy as a “slap” or similar physical “bump”, as the spring driven plunger impacts the stopper of the primary container storing the drug. It is known that such mechanical bumps can also be distracting or disturbing to users of the injectors and can therefore prevent proper dose completion. It is therefore desirable to eliminate such disturbances.
It is further known that the “slap” and “bump” generated by the excessive energy can cause catastrophic effects, such as breakage of the primary container and drug product damage cause by shear load. Furthermore, high force springs can produce high shear rates on the drug product. In some cases, this high shear rate is undesireable.
Accordingly, an autoinjector is needed that can maintain the intended spring force load while reducing the velocity of the plunger before impact with the stopper of the primary container. Such an autoinjector may be potentially more comfortable and safer to use, and applicable to a greater range of drugs.